You can't. Not at the moment, there are a lot of racist white kids out there who have a problem with the game being so orintated towards early 90's black-culture, and were clammering to create a "white C.J." but I doubt it will be happening anytime soon.
Rockstar didn't build any system for easily applying user skins to the game in San Andreas as in their previous titles.
P.S.
You have nothing really to brag about in your signature in terms of your hardware. So don't even bother having a signature, it's taking up bandwidth that could be used for porn. Some people are so greedy...